SEROPREVALENCE OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUSIN DUTCH WEANING PIGS

To determine whether under Dutch field conditions PRRSV infection occu rs in weaning pigs before the finishing period, a cross-sectional stud y was performed on 32 breeding farms to estimate the seroprevalence of antibodies directed against PRRSV in 4- to 5-week-old and 8- to 9-wee k-old pigs, Farms were visited twice within 5 months, and during each sampling an average of 20 sera were randomly collected from a unit of 4- to 5-week-old and a unit of 8- to 9-week-old pigs, The sera (n = 25 68) were tested in the IDEXX-ELISA for the presence of antibodies dire cted against PRRSV, The seroprevalence of PRRSV in 4- to 5-week-old pi gs and 8- to 9-week-old pigs varied between both samplings for each fa rm, The seroprevalence in the younger pigs was significantly higher th an in the older pigs for both samplings (p < 0.05), suggesting the pre sence of maternal antibodies, In addition, a longitudinal study was pe rformed to evaluate the IDEXX-ELISA in detecting maternal antibodies d irected against PRRSV and to determine the rate of decline of these an tibodies in field sera, From serological results of eight litters, an average decay function was computed to quantify the maternal immunity to PRRSV, A seroprevalence in 8- to 9-weeks-old pigs of greater than o r equal to 0.20 was calculated to indicate an active immune response t o PRRSV, In the cross-sectional study in the pigs twenty-three percent of the units with 8- to 9-week-old pigs were considered to have an ac tive serological response against PRRSV. We conclude that most Dutch p igs are seronegative for PRRSV at the start of the finishing period, s ince the results of this study showed that 77% of the units with 8- to 9-week-old pigs had a seroprevalence < 0.20.